The goal of this research is to quantitatively evaluate the level of singular oxygen in enzyme systems such as myeloperoxidase-H2O2-Cl minus, lactoperoxidase-H2O2-Cl minus, NADPH-dependent microsomal lipid peroxidation, and adrenodoxin reductase. A water-soluble substrate which exhibits specific reactivity towards singlet oxygen has been developed. 9, 10-Anthracenedipropanoic acid (1) reacts with singlet oxygen in aqueous solution to yield an isolable endoperoxide. This product is not formed from 1 by reaction with O2. minus, .OH, H2O2 or ground state oxygen. The role of 1O2 in the oxidative damage of membranes is also under investigation. The radical-mediated autoxidation of liposomal membranes and resultant membrane damage will be studied by leakage of trapped 14C-glucose.